The present invention relates to a method and apparatus for performing measurements while drilling for oil and gas in formations beneath the earth's surface, particularly Vertical Seismic Profiling (VSP)-measurements involving the use of a seismic source and sensors, together with a memory and calculation device for storing and processing the received seismic signals.
When extracting oil and gas from formations beneath the earth's surface/sea bed, measurements are performed in wells that are drilled in order to obtain information such as the position of the bit, oil/gas composition/quality, pressure, temperature and other geophysical and geological conditions.
Commonly, well measurements are performed in four phases; 1) when performing drilling operations (MWD--Measurements While Drilling), 2) when logging after the drilling operations have been completed 3) during completion of the well 4) in the production phase.
MWD has primarily been introduced as an aid for direction control when drilling (by position measurements), but gradually this method has also replaced the logging phase.
Furthermore, MWD appears to be the only way of collecting logging information in extended reach wells and in horizontal wells. The application of the MWD equipment has resulted in a reduction of logging costs and it has made it possible to take measurements at locations not previously accessible.
Seismic measurements such as VSP (Vertical Seismic Profiling) have commonly been performed during the logging phase, but measurements of this type have also been employed during the completion phase and the production phase of the wells.
For a long time, there has been a demand for performing VSP-measurements in the drilling phase, referred to as MWD-VSP, as it is assumed that this will reduce costs, but the equipment necessary for performing such measurements has not been available until now.
VSP-measurements are in particular employed to determine the vertical travel time through the ground in a frequency band that is characteristic for seismic records taken at the surface of the earth. By these means, detailed measurements performed in the well may be intergrated with seismic measurements performed at the surface of the earth. In VSP-measurements, the source will be located close to the surface or in the well, with corresponding sensors in the well or close to the surface. Today, data from VSP-measurements are collected by means of a source located close to the sea surface, and with the recording system connected to the logging cable. The geometry of the VSP-measurements may further be used for recording reflected signals that are weak, and that represent structural surface elements such as stratigraphic borders/fault planes in the close (max. one km.) vicinity of the well (primarily under, but also to the side of and above the well). MWD-VSP is equivalent to a VSP logged in the traditional manner, and may at the same time be used as an aid for direction/position control when performing the drilling operation.
There is a previously known method based upon "passive" source MWD-VSP where the drill bit serves as the source, and where the recording equipment is placed on the sea bed.
This method is somewhat dated, and today it still represents the sole method developed for industrial applications. The method was marketed as a successor to traditional VSP on its introduction 1988, but it has proved to be encumbered with so many substantial limitations that little data has been collected under operational conditions that characterise drilling in the North Sea. The companies that invented the method point out that the most essential condition for generating signals that are sufficiently strong, is to use drill bits of the "roller cone" type. Thus, the method is not considered to be of current interest for use in conjunction with other types of drill bits, and therefore most of the delineation and production wells in the North Sea cannot be served by this method. The signal transmitted and the time of first arrival are often partly degraded to a relative accuracy of 5 msec and to an absolute accuracy of approximately 10 msec. These uncertainties are not acceptable when performing measurements in delineation and production wells. The examples given by contractors relating to the use of the reflected signals, have not been considered as usable, and the details in the reflected signals that are critical in a drilling operation and in the interpretation of the surface seismic after the drilling operation has been completed are at a level substantially below the noise level present in the records.
The disadvantages characterized by the passive source method mentioned above, may be avoided by making the source strong and controllable. Among other things, there have been suggestions to apply sources driven by the circulating drilling mud. Also, tests of the use of a telemetry pulser as a source have been performed. Further, tests have been performed employing special sources that are stronger than the telemetry pulser. Meanwhile, it is uncertain whether such a source will be harmful to the well and whether the energy level in the mud will be sufficient for driving more powerful sources while still meeting the demand for the energy needed in the drilling operation.